Field of the Invention
The present invention relates to a combined sensor that detects acceleration and an angular velocity and, more particularly, to a technique effectively applied to the configuration of an inertial sensor that includes a function for detecting presence or absence of a failure or an abnormality and provides high reliability.
Background Art
According to the development of the semiconductor micromachining technique and the integrated circuit technique, a sensor of a type called MEMS (Micro Electro Mechanical Systems) that detects inertia has been developed. In particular, application of the sensor device to the automobile field is in progress.
For example, in an angular velocity sensor, because the MEMS sensor has a characteristic that the MEMS sensor can be manufactured more inexpensively and in a smaller size than a Fiber Optic Gyro in which an optical fiber in the past is used, the MEMS sensor is being widely applied to new vehicle control systems such as a electronic stability control system and a rollover detection system for consumer automobiles.
In an acceleration sensor, the MEMS sensor has a characteristic that the MEMS sensor can be manufactured in a small size, for uses such as collision detection and suspension control, there has been developed a new application method for attaching a plurality of modules in various places of a vehicle and realizing fine control.
Further, in recent years, an activity for advancing a cost reduction is in progress according to “combining” for realizing a plurality of physical quantity detections with one sensor module or one detecting element chip. This has enabled application of the vehicle control system mounted only on a luxury car to a popular car.
However, in these sensors for vehicle mounted applications, it is likely that a failure of the sensors leads to a severe accident. Therefore, it is desirable to not only reduce a failure occurrence rate but also provide a “failure detecting function” for, if a sensor failure should occur, immediately notifying a host system of the failure. However, such a failure detecting function is not always necessary for an inertial sensor for so-called electronic devices such as a portable information terminal and a game machine. Therefore, the addition of such a function for detecting a failure causes an increase in manufacturing costs and adjustment costs for the sensors for vehicle mounted applications.
For example, JP-A-2005-114394 (Patent Literature 1) discloses, in an inertial sensor including a detecting element 6, a configuration for applying an alternating-current bias signal voltage to a component preceding a synchronous demodulator 24, specifically, to the detecting element 6 to thereby detect a failure such as breaking of wire or a ground fault that could occur in the detecting element 6 or a detection circuit 27. To solve the problem explained above, in particular, in order to reduce manufacturing costs, Patent Literature 1 discloses, in an inertial sensor that includes an angular velocity sensor including a self-excited oscillation loop circuit, an excitation frequency of a self-excited excitation loop signal voltage of which is equal to or lower than 50 kHz, and detects acceleration and an angular velocity, a configuration for using the self-excited oscillation loop signal voltage of the angular velocity sensor as the alternating-current bias signal voltage of the acceleration sensor. In this technique, a circuit for generating the alternating-current bias signal voltage of the inertial sensor is made unnecessary. Therefore, it is considered that manufacturing costs can be reduced.
JP-A-2011-95104 (Patent Literature 2) discloses, in a capacitance type sensor, a configuration for electrically separating a capacitative element C1 and a capacitive element C2 that detect a capacitance change due to displacement of a detecting element and a capacitive element C3 and a capacitive element C4 that configure a forced vibration generating section. That is, a voltage signal for causing the capacitive element C3 and the capacitive element C4, which generate forced vibration, to generate an electrostatic force is applied to a movable section of the detecting element. With this configuration, the movable section is physically displaced and this displacement is detected through the capacitive element C1 and the capacitive element C2. Consequently, it is possible to detect a mechanical failure of the detecting element such as sticking and breakage of a beam. In the configuration, a voltage signal for causing forced vibration for a diagnosis in the detecting element is not superimposed. The respective signals are given from different elements. Therefore, there is no crosstalk of a diagnosis signal with a detection signal and offset fluctuation does not occur. This is effective in terms of preventing a wrong diagnosis of the sensor.
The inventor examined the related arts of Patent Literature 1 and Patent Literature 2. As a result, problems explained below were clarified.
For example, in the configuration for using the self-excited oscillation loop signal voltage of the angular velocity sensor as the alternating-current bias signal voltage of the inertial sensor as in the technique described in Patent Literature 1, a configuration for making a circuit for generation of the alternating-current bias signal voltage of the inertial sensor unnecessary is realized. However, since the movable section of the detecting element is not actually moved, a sensitivity change or a resonant frequency change due to sticking of the detection element or breakage of a beam cannot be detected. That is, even if the sticking occurs and the inertial sensor falls into a state in which the inertial sensor cannot detect any inertia change, this state cannot be recognized and a misrecognition that “the inertial sensor is in a normal state” occurs.
In the configuration for giving forced vibration and detecting a mechanical failure such as sticking as in the technique described in Patent Literature 2, if a resonant frequency of a target detection element is high, that is, under a condition that the movable section is supported using a hard beam, there are disadvantages. For example, disadvantages occur in that manufacturing costs are sacrificed or costs of the detecting element are sacrificed, for example, in order to generate a strong electrostatic force for moving the detecting element, (1) the driving voltage signal for causing vibration is set to a high voltage and (2) a large area of an electrode is secured to increase the capacitance of the capacitive elements. In particular, such conditions occur in an acceleration sensor having a hard beam, that is, an acceleration sensor having a high range, for example, detection range such as several tens G (1G is gravitational acceleration of 9.8 m/s2) to several hundred G. As an application example of the acceleration sensor, an acceleration sensor for detection of large acceleration such as “collision detection” for detecting timing for actuating an air bag is relevant.